System and method for measuring physical, chemical and biological stimuli using vertical cavity surface emitting lasers with integrated tuner

ABSTRACT

An optical sensor diagnostic system utilizing a tunable Vertical Cavity Surface Emitting Laser (VCSEL) that incorporates an integrated MEMS tuning mechanism provides variable wavelength light into an optical fiber with improved wavelength scanning speed and greater simplicity of construction. Sensors, such as Bragg gratings, are disposed along the fiber in the light path. Each sensor reflects or transmits light exhibiting a characteristic amplitude feature with respect to wavelength, the wavelength position of which is affected by an environmental stimulus imposed thereon. The power of the reflected light is converted to an electrical signal by a simple detector and monitored by circuitry that detects changes in reflected power and provides output signals indicative of the environmental stimulus for each sensor.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

FIELD OF THE INVENTION

[0003] This invention relates to systems using vertical cavity, surfaceemitting lasers (VCSELs) having integrated MEMS(micro-electromechanical) wavelength tuner to interrogate opticalsensors, including fiber and planar Bragg gratings, etalons,characteristic absorption or reflection sensors such as bandgapsemiconductors and surface plasmon resonance sensors sensitive tophysical, chemical and biological stimuli and, more particularly, tospecific system configurations for use with such Bragg grating, etalon,absorption/reflection and surface plasmon resonance sensing devices.

BACKGROUND OF THE INVENTION

[0004] Fiber optic sensors employing measurements of the shift ofwavelength position of a sensor spectral peculiarity (maximum, minimum,slope or some other function) under the influence of a physical stimulusare well known to those skilled in the art. The examples of such sensorsinclude Bragg grating-based strain, pressure, temperature and current(via the associated magnetic fields) sensors, surface plasmon resonance(SPR) biological and chemical sensors, semiconductor absorption bandedgebased fiber-optic sensors and Fabry-Perot (FP) etalon pressure,temperature and sensors. The utilization of such sensors has beenretarded in the marketplace because of many well known problems,including the susceptibility of simple, inexpensive sensing systems tooptical noise and the great expense of most of the solutions found toovercome said susceptibility. It will be revealed that combining a newtype of laser, a vertical cavity, surface emitting laser (VCSEL) with anintegrated microelectromechanical (MEMS) tuning mechanism, as aninterrogating instrument with sensors of many different types willenable new, less expensive and more reliable class of optical sensorsystems.

[0005] A Bragg grating is a series of optical elements that create aperiodic pattern of differing indices of refraction in the direction ofpropagation of a light beam. A Bragg grating is formed in an opticalfiber by means of exposing ultraviolet sensitive glass (usuallygermanium doped fiber) with an ultraviolet (UV) beam that variesperiodically in intensity, usually accomplished by means of aninterference pattern created by a phase mask or split beam, such as witha Lloyd's mirror apparatus. Planar Bragg gratings are created byexposing a “photoresist” of any of a number of types through a phaseshift or other type of mask, or they can be written directly with anelectron beam. Light reflections caused by the periodic index ofrefraction pattern in the resulting grating interfere constructively anddestructively. Since the refractive index contrast between UV-exposedand unexposed sections of fiber is small but the number of sections isvery large, the reflected beam narrows its spectrum to a very sharppeak, as narrow as a fraction of a nanometer in spectral width. It canalso be arranged by means of a phase shift design that the reflectedpeak can contain within it an even narrower “valley” of absorption, asnarrow as a few picometers in spectral width. Conversely, thetransmitted portion of the light beam exhibits complimentary spectralpower characteristics, i.e., a broader valley with a narrower peakwithin it.

[0006] It is known that Bragg gratings patterned into optical fibers orother waveguides may be used to detect physical stimuli caused byvarious physical parameters, such as, for example, strain, pressure,temperature, and current (via the associated magnetic fields) at thelocation of the gratings,-such as is described in U.S. Pat. Nos.4,806,012 and 4,761,073 both to Meltz, et al; U.S. Pat. No. 5,380,995issued to E. Udd; U.S. Pat. No. 6,024,488 issued to J. Wu; and thepublication authored by Kersey, A. D., et. al. [10^(th) Optical FiberSensors Conference, Glasgow, Oct 1994, pp.53-56]. Generally, in such asensor, the core and/or cladding of the optical fiber (or planarwaveguide) is written with periodic grating patterns effective forselectively reflecting a narrow wavelength band of light from a broaderwavelength band launched into the core (waveguide layer in thewaveguide). The spectral positions of sharp maxima or minima in thetransmitted and reflected light intensity spectra indicate the intensityof strain, temperature, pressure, electrical current, or magnetic fieldvariations at the location of the grating. The mechanism of the spectralposition changes lies in changes either the in grating period or theindices of refraction, or both, which can be affected by variousenvironmental physical stimuli, such as temperature and pressure.Frequently, more than one stimulus or physical parameter affects thesensors at the same time, and compensation must be designed into thesensor or the measurement technique for all the variables but one, whichcan be accomplished by many physical, optical and electronic techniquesknown in the art. The typical sensitivity limits of fiber gratingsensors in the current art are about 0.1° C. and/or 1 microstrain,respectively. Advantages of a spectral shift method of sensorinterrogations include the high accuracy of wavelength determination(akin to the advantages of measuring electrical frequency instead ofmagnitude) and immunity to “optical noise” due to fluctuations in fibertransmission amplitude (microbending losses, etc.). It also allows themultiplexing of many sensors on the same fiber via wavelength dependentmultiplexing techniques (WDM), e.g., dividing the total wavelength bandinto sections dedicated to individual sensors.

[0007] The precision, dynamic range and multiplexing capabilities of theall optical sensor interrogation techniques are defined in part by thespectral power of the light source, especially in cases in which abroadband source is used. The LEDs, SLDs (superluminescent diodes) andvarious lamps usually used provide spectral power that can be too littlewhen divided into nanometer-sized segments. This limits criticalparameters such as the magnitude of the reflected peak available to theoptical sensor, causing lower than desirable signal to noise ratios.Another technique, the use of a conventional laser diode tuned withmotorized external cavity, electrical current or temperature mechanismsis more effective because all the power of the laser is contained in anarrow beam as it is tuned across the spectrum. Several techniques havebeen proposed: see for example Froggatt, (U.S. Pat. No. 5,798,521); theuse of a conventional laser diode tuned with electrical current has beenproposed by Dunphy et. al. (U.S. Pat. No. 5,401,956); and the use of atunable fiber laser has been proposed by G. A. Ball et. al. [J. ofLightwave Technology, vol. 12, no. 4, April 1994 p 700]. When using ascanning laser technique, an inexpensive detector and electronics systemsimply determines the wavelength at the peak (or null) of the reflected(or transmitted) light intensity against a known wavelength reference.However, past art approaches are generally too expensive, too slow, toounstable or too inaccurate to have a wide range of practicalapplications. Laser diodes tuned with current, while inexpensive andfaster than thermal methods, suffer from narrow tuning wavelength spans,which limits practical applications to only time division-multiplexed(TDM) Bragg sensors. Such lasers are completely unusable in surfaceplasmon or semiconductor absorption edge shift sensors. The broadbandlight source method utilizes an inexpensive light source, but requires aspectrometer to read the signals (an optical spectrum analyzer may costas much as $35,000). It is most practical when many sensors aremultiplexed on the same fiber. Still, spectrometers are temperamentaland not well suited to field use. The lasers tuned with externalcavities that are now in use, on the other hand, typically are moreexpensive than spectrometers, but have the advantage of using aninexpensive detector. In addition, such lasers are typically slow totune, such as 100 nm/sec, and may be even more delicate thanspectrometers. Scanning (or tuning) speed is especially important inapplications in which absorption and polarization related noise aresignificant because of negative effects on the signal to noise ration(SNR). Mass produced MEMS-tunable VCSELs, configured as sensinginstruments, are expected to cost at least an order less than prior artlasers and be at least two orders of magnitude faster than prior artlasers.

[0008] Surface plasmon resonance-based sensors for biological and/orchemical monitoring are well known to those skilled in the art. Surfaceplasmon waves are electromagnetic waves that may exist at the boundarybetween a metal and a dielectric (hereinafter referred to as the“sample”). Such waves can be excited by light that has its electricfield polarized parallel to the incident plane (i.e., transversemagnetic (TM) polarized). When the parallel component of the propagationconstant of the incident light equals the real part of the surfaceplasmon wave propagation constant, the incident light resonantly excitesthe surface plasmon waves, and a fraction of the incident light energyis transferred or dispersed to surface plasmon resonance (SPR). Thisdispersion of energy depends on both the dielectric constant of themetal and that of the sample in contact with the metal. By monitoringthe resonance wavevector of the metal/sample interface, the dielectricconstant of the sample (gas or solution) may be obtained. Alternatively,if the sample is contaminated by a chemical species, dielectric constantmeasurements may provide the concentration of the chemical species inthe sample. The typical SPR spectral minimum is at least two orders ofmagnitude wider than the typical Bragg grating minimum or maximum.

[0009] Traditionally, SPR has been measured using the Kretschmannconfiguration (Kretschmann and Raether, Z. Naturforsch. Teil A23:2135-2136, 1968). In this configuration, a thin layer of highlyreflective metal (such as gold or silver) is deposited on the base of aprism. The metal surface is then contacted with the sample, and the SPRreflection spectra of the sample is measured by coupling TM polarized,monochromatic light into the prism and measuring the reflected lightintensity as a function of the angle of incidence. The angle of minimumreflective intensity is the resonance angle at which maximum couplingoccurs between the incident light and the surface plasmon waves. Thisangle, as well as the half-width of the resonance spectrum and theintensity at the angle of minimum reflective intensity, may be used tocharacterize or sense the sample that is in contact with the metalsurface (Fontana et al., Applied Optics 27:3334-3339, 1988).

[0010] Optical sensing systems have been constructed based on theKretschmann configuration described above. Such systems utilize thesensitivity of SPR to changes in the refractive indices of both bulk andthin film samples, as well as to changes in the thickness of thin films.These systems, in conjunction with appropriate chemical sensing layers,have led to the development of a variety of SPRbased chemical sensors,including immunoassay sensors (e.g., Liedberg et al., Sensors andActuators 4:299-304, 1983; Daniels et al. [Sensors and Actuators15:11-17, 1988]; Jorgenson et al., [IEEE/Engineering Medicine andBiology Society. Proceedings 12:440-442, 1990]), gas sensors (e.g.,Liedberg et al., ibid, Gent et al., [Applied Optics 29:2843-2849,1990]), and liquid sensors (e.g., Matsubaru et al., [Applied Optics27:1160-1163, 1988]). An SPR sensor usually utilizes the wavelength ofminimum amplitude as a function or angle of reflection. However, theshape of the minima can be modified if an additional polarizer and phaseplate (or retarder) are introduced between the sensor and detector atsome predetermined angle with respect to the polarization of lightilluminating the SPR sensor (Homola J., et al, Sensors and Actuators B,B51 (1-3), August 1998, p.331, Kabashin A. V et al, Sensors andActuators B, B54 (1-2), January 1999, p.51). This modification is due tophase and polarization peculiarities near the surface plasmon resonanceexcitation conditions. Moreover, minima can be transformed into maxima(Homola, ibid), which has the potential for increasing the resolution ofSPR sensors

[0011] While the Kretschmann configuration for SPR-based chemicalsensors offers significant sensitivity, their relatively large size hasseverely restricted their application. An optical fiber sensor thatutilizes SPR to detect a material in contact with the sensor andutilizes incident light having multiple wavelengths as the excitationenergy is described by Jorgenson, et al. (U.S. Pat. No. 5,359,681).While being small and considerably less expensive than the non-waveguideoptical sensor, it is at least an order of magnitude less sensitive. Thereason for this drop in sensitivity is obvious—in a non-waveguideoptical scheme with 5000 pixels, the SPR minimum is read by at least2000 pixels. However, in the fiber optic scheme with a spectrometer as areadout instrument (wavelength resolution not better than 0.1 nanometerand SPR minimum spectral width around 60 nm) the SPR minimum will becharacterized by 600 pixels at most, leading to less preciseinterpolations to locate the minimum, and hence changes in thewavelength position of the minimum. Conversely, the use of inexpensivetunable lasers with sub-nm wavelength resolution as light sources infiber systems will eliminate the expensive spectrometer and yieldprecision at least comparable to that of the non-waveguide opticalsystem. The illustrative problem that arises in the broadband lightsource/spectrometer configuration, specifically a lack of opticalintensity per measurement point, has, in the case of SPR sensors, theadditional undesirable attribute of overheating of the sensor. If thetotal light intensity of the broadband light source is increased tocompensate for the small intensity available to each pixel in thespectrometer charge coupled device (CCD) array, overheating of the SPRsensor will occur because approximately half the incident light isdissipated in heat in the metal layer whether it contributes to theusable signal or not. Heating is not only harmful to the biologicaland/or chemical sample under test, but also can induce refractive indexchanges in the fiber and/or sample, causing much larger variations inthe SPR wavevector than the perturbation to be detected. Very fasttuning lasers, having very narrow emission spectra, are ideal to addressthis problem, since the total intensity illuminating the sensor at anygiven time will be almost exactly equal to the intensity of thereflected light available to detect the change in stimulus.

[0012] An illustrative example of a characteristic absorber/reflectormaterial is a semiconductor. A semiconductor means that comprises anoptical sensor with optical-wavelength-dependent characteristics thatmay vary as a function of a physical parameter such as temperature iswell known to those skilled in the art (see, for example patents, issuedto Christenson (U.S. Pat. No. 4,136,566) or Quick et al. (U.S. Pat. No.4,355,910). The optical-wavelength-dependent characteristic(semiconductor absorption band edge) is usually monitored by present artmethods in one wavelength band, in which case measurements areintensity-dependent, or in two wavelength bands, after which a ratio ofintensities is taken. In both cases, the sensitivity and accuracy ofsuch sensor systems are low and more-or-less sensitive to optical noise(microbending, etc.). Scanning very rapidly through the wholesemiconductor transmission intensity slope related to the forbidden bandedge using a tunable VCSEL with high wavelength resolution will providethe opportunity for mathematical enhancement of the sensitivity of suchsensors by at least in order of magnitude. Broadband lightsource/spectrometer configurations are not suitable for reasons similarto those described in [0011]. The high total illumination intensity willcause self-heating of the sensor, which is crucial especially fortemperature sensors. Reducing the illumination intensity, on the otherhand, will cause uncertainty due to photodetector dark noise and othersources of optical noise.

[0013] Fiber etalon-based sensors are well known to those skilled in theart (see, for example, U.S. Pat. No. 5,646,401 issued to E. Udd).Etalons consist of two mirrored surfaces that may be internal orexternal to the optical fiber. The reflectivity of an etalon is definedby interference between light waves reflected from first and secondmirrors. The advantages of etalon-based pressure, temperature and/orstain sensors include the low cost of etalons and very high sensitivity.However, with broadband light sources used for interrogation,measurements that are intensity based or count interference fringes arevery susceptible to optical noise or other technical problems (e.g.,losing count of the fringes), to the point of being impractical. Thesole practical, self-calibrating system uses an opticalcross-correlating interferometer as a detector, also an expensivetechnique (see, for example, U.S. Pat. Nos. 5,202,939 and 5,392,117 bothissued to Belleville, et al.).

[0014] A new kind of laser, a vertical cavity surface emitting laser(VCSEL), has recently been invented. Generally, VCSELs are madecompletely with waferlevel processing and the chips emit from thedirection of the broad surface of the wafer, rather than having to becleaved out of the wafer in order to have an exposed pn junction edgefrom which to emit, as in older art. This enables another benefit to bedesigned into the wafer structure—tunability. This is done withmicromachining (MEMS) technology by placing a stack of optical layers,forming a mirror, in front of the emitting surface in such a way thatthe stack can be varied in its distance from the emitting surface bypiezoelectric, magnetic, electrostatic or some other microactuatingmeans. The groups of C. J. Chang-Hasnain (US Patent, [IEEE J. onSelected Topics in Quantum Electronics, V 6, N 6, November 2000, p.978]), J. S. Harris Jr. (U.S. Pat. No. 5,291,502, [Appl. Phys. Lett. 68(7), February 1996 p. 891]), and Vakhshoori [Electronics Letters, May1999, V. 35, N.11 p. 900] have shown the potential for making tunableVCSELs with MEMS tuning mechanisms with wide tuning ranges and fasttuning speeds. Tunable VCSELs are relatively simple to manufacture,exhibit continuous mode-hop-free tunability over a wide spectrum, andpotentially offer orders of magnitude lower cost as compared to priorart tunable lasers or optical spectrometers. Integrated, MEMS-tunableVCSELS make possible truly affordable and accurate optical sensorsystems by combining low cost detectors and low cost excitation sources,one or the other of which is very expensive in the prior art systemswith the accuracy and resolution to be viable commercially. In additionto the orders of magnitude lower cost of source/detector combinations,lower cost sensor will become available because of the orders ofmagnitude greater tuning speed.

BRIEF SUMMARY OF THE INVENTION

[0015] The present invention provides a means of optical wavelengthscanning Bragg grating, characteristic absorber/reflector, etalon andsurface plasmon resonance sensors of all types with integrated,MEMS-tunable VCSELs in order to measure various physical parameters atseveral orders of magnitude lower cost than prior art, with the addedbenefits of enhanced accuracy, ruggedness and reliability.

[0016] In more detail, the present invention provides, as anillustrative embodiment, a diagnostic system which interfaces withoptical fibers or optical waveguides having Bragg grating or other typesof sensors as described herein, embedded therein for the determinationof static and dynamic values of various physical, chemical or biologicalparameters, and, further, to provide means of guaranteeing wavelengthaccuracy during the scanning cycle.

[0017] In accordance with one aspect of a preferred illustrativeembodiment of the invention, an optical sensor diagnostic systemincludes an integrated MEMStunable VCSEL for providing awavelength-tunable light in response to a voltage or other controlsignal, the tunable light being launched into an optical waveguide. Atleast one optical sensor, disposed in the path of the tunable light,provides a reflected light having an associated local amplitude minimum,maximum or slope. The said local amplitude maximum could contain one ormore local amplitude minimums inside said local amplitude maximum, whilesaid local amplitude minimum could contain one or more local amplitudemaximums inside said local amplitude minimum. The wavelength at saidminimum, maximum or slope of amplitude varies in response to anenvironmental stimulus imposed upon the corresponding sensor. Thetunable VCSEL individually illuminates each of the sensors throughoutits associated wavelength band of an amplitude minimum, maximum orslope. An optical circulation device, disposed in the path of thetunable light between the tunable VCSEL and the sensors, isolates thetunable VCSEL from the reflected light and directs the reflected lightfrom each of the sensors to the optical detector means, disposed fordetecting the reflected light and for providing an electrical detectionsignal indicative of the power of the reflected light. A tuningcontroller provides a variable voltage or other signal to the tunableVCSEL indicative of the desired wavelength of the tunable light. Asignal processor responsive to the electrical detection signalinterprets a shift in the wavelength of the magnitude minimum, maximumor slope due to the environmental stimulus, and provides a signalindicative of said stimulus.

[0018] According to another aspect provided by an illustrativeembodiment of the present invention, an optical sensor diagnostic systemincludes an integrated MEMS-tunable VCSEL for providing awavelength-tunable light in response to a voltage or other controlsignal, the tunable light being launched into an optical waveguide. Atleast one optical sensor, disposed in the path of the tunable light,provides a transmitted light having an associated local amplitudeminimum, maximum or slope. The said local amplitude maximum couldcontain one or more local amplitude minimums inside said local amplitudemaximum, while said local amplitude minimum could contain one or morelocal amplitude maximums inside said local amplitude minimum. Thewavelength at said minimum, maximum or slope of amplitude varies inresponse to an environmental stimulus imposed upon the correspondingsensor. The tunable VCSEL individually illuminates each of the sensorsthroughout its associated wavelength band of an amplitude minimum,maximum or slope. An optical isolation device, disposed in the path ofthe tunable light between the tunable VCSEL and the sensors, isolatesthe tunable VCSEL from the reflected light. The light transmittedthrough the said at least one optical sensor is directed by an out-goingfiber to the optical detector means, disposed for detecting thetransmitted light and for providing an electrical detection signalindicative of the power of the transmitted light. A tuning controllerprovides a variable voltage or other signal to the tunable VCSELindicative of the desired wavelength of the tunable light. A signalprocessor responsive to the electrical detection signal interprets ashift in the wavelength of the magnitude minimum, maximum or slope dueto the environmental stimulus, and provides a signal indicative of saidstimulus.

[0019] In accordance with one aspect of a preferred illustrativeembodiment of the invention, the said optical sensors are of reflectiveBragg grating type. The sensors reflect light, having maxima or minimainside the maxima at a different reflection wavelength for each sensor,which vary their spectral positions due to an environmental stimulus,such as strain, pressure, temperature, electrical current or magneticfield imposed thereon.

[0020] In accordance with another aspect of a preferred illustrativeembodiment of the invention, the said optical sensors are oftransmittive Bragg grating type. The sensors transmit light, havingminima or maxima inside the minima at a different transmissionwavelength for each sensor, which vary their spectral positions due toan environmental stimulus, such as strain, pressure, temperature,electrical current or magnetic field imposed thereon.

[0021] In accordance with further aspect of a preferred illustrativeembodiment of the invention, the said optical sensors are of reflectiveetalon type. The sensors reflect light, having maxima, minima or maximaand minima at a different reflection wavelength for each sensor, whichvary their spectral positions due to an environmental stimulus, such asstrain, pressure, temperature, electrical current or magnetic fieldimposed thereon.

[0022] In accordance with further aspect of a preferred illustrativeembodiment of the invention, the said optical sensors are oftransmittive etalon type. The sensors transmit light, having maxima,minima or maxima and minima at a different transmission wavelength foreach sensor, which vary their spectral positions due to an environmentalstimulus, such as strain, pressure, temperature, current or magneticfield imposed thereon.

[0023] In accordance with further aspect of a preferred illustrativeembodiment of the invention, the said optical sensors are of reflectiveSurface Plasmon Resonance type. The sensors reflect light, having maximaor minima at a different reflection wavelength for each sensor, whichvary in their spectral positions due to an environmental stimulus, suchas temperature, biological or chemical stimuli imposed thereon.

[0024] In accordance with further aspect of a preferred illustrativeembodiment of the invention, the said optical sensors are oftransmittive Surface Plasmon Resonance type. The sensors transmit light,having maxima or minima at a different reflection wavelength for eachsensor, which vary in their spectral positions due to an environmentalstimulus, such as temperature, or biological and chemical stimuliimposed thereon.

[0025] In accordance with further aspect of a preferred illustrativeembodiment of the invention, the said optical sensors are of acharacteristic absorber/reflector type. Said characteristicabsorber/reflector sensors, disposed in the path of the tunable light,provide a transmitted light having an associated local amplitude slopeor local amplitude minimum, the wavelengths of which vary their spectralposition due to an environmental stimulus, such as temperature, imposedthereon. This embodiment could be also realized in reflection mode, withmultiple sensors coupled off the main fiber, if the reflective means isdisposed in the light path such that the light is double-passed throughthe each of the sensors by means of a mirror and time domainmultiplexing is utilized. In the case of an absorber/reflectorexhibiting a minimum, wavelength division multiplexing can be utilizedto the degree the width of the minimum allows as a fraction of theavailable tuning spectrum. The isolator in this realization of thepresent embodiment must be replaced by a circulator means.

[0026] The illustrative embodiments of the invention provide low cost,workable, practical diagnostic systems which function in cooperationwith remote optical fiber sensor systems to measure static and dynamicstrain, pressure, temperature, electrical currents and magnetic fieldsas well as acoustic or vibratory perturbations of items or structuresand chemical and biological parameters. The remote sensors may bedisposed on structures made of metal, plastic, composite, or any othermaterials that expand, contract, or vibrate, or the sensors may beembedded within such structures or immersed in liquids or gasses. Theembodiments also provide a wavelength-tunable VCSEL, tunable smoothlyand monotonically, and in particular, linearly or sinusoidally tunablewith time. The embodiments further provide individual illumination ofeach sensor, thereby allowing all the tunable VCSEL power to be residentin a single narrow wavelength band at any instant in time. As a result,the reflected or transmitted light from each optical sensor has a highintensity, thereby providing a signal-to-noise ratio of such reflectedor transmitted light that is much greater than systems that illuminateall sensors at the same time using a broadband source. Ultra-fine tuningof tunable VCSELs to a few parts per million will allow another order ofmagnitude increase in precision due to higher resolution and improvedcomputational methods and statistical processing. The very low mass ofthe MEMS tuning mechanisms allow very high tuning speeds with very lowhysteresis, providing the ability to average out optical noise in thesensor systems with many data points and allowing very close spacing ofdata in wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The foregoing and other objects, features and advantages of thepresent invention will become more apparent in light of the followingbrief description of exemplary embodiments thereof as illustrated in theaccompanying drawings, of which:

[0028]FIG. 1 is a schematic drawing of an illustrative VCSELincorporating one example of an integrated MEMS (micro-electromechanicalmachined system) tuning mechanism in the form of a cantilevered mirrorand optional lens;

[0029]FIG. 2A is a schematic block diagram of a first state of anillustrative, exemplary non-limiting sensor diagnostic system capable ofdetermining the value of static and dynamic physical, chemical orbiological physical stimuli in a reflection mode employing a tunableVCSEL as an optical excitation source and a photodiode or similar simpledevice as a detector;

[0030]FIG. 2B is a schematic block diagram of a first state of anillustrative, exemplary non-limiting sensor diagnostic system capable ofdetermining the value of static and dynamic physical, chemical orbiological stimuli in a transmission mode, employing a tunable VCSEL asan optical excitation source and a photodiode or similar simple deviceas a detector;

[0031] FIGS. 3A-3J are a series of illustrative, exemplary graphsshowing time-varying tuning control signals, Vt, applied to a tunableVCSEL and the resulting spectral power transmission and/or reflectionsignals produced by the sensor;

[0032]FIG. 4 is a graph of an illustrative, exemplary transmissionoptical power profile of a sensor employing Surface Plasmon Resonance toproduce a shift in the wavelength position of the spectral power minimumin response to a physical, chemical or biological stimulus;

[0033]FIG. 5A is an illustrative, exemplary diagram of the spectralpower transmission of a characteristic absorber/reflector in the form ofa bandgap semiconductor showing a shift in wavelength due to an increasein temperature; and

[0034]FIGS. 5B and 5C are, respectively, illustrations of the increasesin the accuracy of wavelength shift determination with the knowledge ofthe first and second derivatives of the power transmission spectra.

BEST MODES FOR CARRYING OUT THE INVENTION

[0035]FIG. 1 is an illustrative schematic drawing of a VCSELincorporating one example of an integrated MEMS (micro-electromechanicalmachined system) tuning mechanism in the form of a cantilevered mirrorand optional lens. Substrate chip 30 has fabricated upon it, when inwafer form, a multilayer stack of materials forming the light emittingVCSEL 31 and the tuning components consisting of mirror stack 32,actuator and structural means 33 that change the tuning cavity length38, diffractive optical lens 34 (optional), capacitive cantileverposition monitor 35 (optional) and light beams emitted in directions 36and/or 37. If either mirror 32 or bottom of stack 31 is opaque, light isemitted only in one direction 37 or 36 respectively. Emission in bothdirections is possible if both 32 and 31 are partially transparent. Thisarrangement provides the simplest means for optical power monitoringwith a photodiode 38 for the purpose of spectral power uniformitycontrol. Electrical connections are not shown for simplicity.

[0036]FIG. 2A shows an example illustrative embodiment of a diagnosticsystem 40 provided in accordance with one aspect of this invention. Inmore detail, FIG. 2A is a schematic block diagram of a first state of anillustrative, exemplary sensor diagnostic system capable of determiningthe value of static and dynamic physical, chemical or biologicalphysical stimuli in a reflection mode employing a tunable VCSEL as anoptical excitation source and a photodiode or similar simple device as adetector. Preferred embodiment diagnostic system 40 includes aMEMStunable VCSEL 30, a fiber light coupling means 41, an opticalcirculator 47, a wavelength reference 43, exterior fiber 44 and couplingmeans to the sensor or sensor array 45, a photodetector 48, and acontrol block 49. In said illustrative example, a tunable VCSEL 30 isassembled with necessary means to fiber couple 41 the emitted light,provide an accurate wavelength reference 43, and couple the VCSELassembly 40 to an external fiber 44 to convey the laser light to anoptical sensor or sensor array 45 in a reflection mode. A coupler orcirculator 47 must be provided to divert the optical signal reflectedfrom the sensor 45 to the photodetector 48, the electrical signal fromwhich is relayed to the control block circuitry 49 and externalelectronic circuitry as required. A circulator also provides thefunction of isolating the VCSEL from back-reflected light. If a coupleris used to divert the light to the detector, a separate isolator must beincorporated between it and the laser. The control block 49 may or maynot control the laser temperature via a thermoelectric element or othermeans and may or may not adjust the laser power output according to asignal from a monitor photodiode 38, as required.

[0037] In this embodiment, the tunable VCSEL 30 provides awavelength-tunable light in response to a tuner control signal providedby control block 49. This tunable light provided by tunable VCSEL 30 islaunched into an optical waveguide 44 such as an optical fiber. A sensoror sensor array 45, in this embodiment a Bragg grating sensor array,providing at least one optical sensor, is disposed in the path of thetunable light. The sensor array 45 includes individual Bragg gratingsthat each reflects light having different, non-overlapping, associatedamplitude reflection maxima at individual reflection wavelengths,spectrally distinguishable one from the other. In the exemplaryembodiment, the wavelength position of the amplitude maximum reflectedby each of the Bragg gratings in the array 45 varies in response to aphysical stimulus or perturbation imposed on the corresponding sensor.

[0038] The tunable VCSEL 30, by continuously scanning its outputspectrum, individually illuminates each of the sensors in turn withinthe sensor array 45 in a wavelength band including the wavelength ofmaximum or minimum reflection associated with each sensor. An opticalisolation and directing device such as optical circulator 47 is disposedin the path of the tunable light between the tunable VCSEL 30 and thesensor array 45. The circulator efficiently isolates the tunable VCSELfrom light reflected by the sensor array 45 and diverts the reflectedsignals to a simple and inexpensive optical detector 48, such as aphotodiode, disposed in the path of the light. The detector 48 providesan electrical detection signal indicative of the power of the reflectedlight that is directly related to the wavelength through the tuningcontrol signal and the wavelength reference 43, if utilized. The seriesof optical signals obtained during a scanning cycle can contain one ormore absorption or reflection bands from one or more wavelengthreference devices 43 that can be incorporated for additional wavelengthaccuracy. Said reference signals do not change in wavelength positionwith any of the external stimuli measured by the sensors, and can berelated in time to the tuning control signal.

[0039] Control block 49 responds to the electrical detection signal fromthe photodetector 48 in the example embodiment by calibrating a variablevoltage or other tuning signal for the tunable VCSEL 30 to thewavelengths of the wavelength references, and providing said tuningsignal to said VCSEL. Control block 49 may also include a signalprocessor responsive to the electrical detection signal for detecting ashift in the wavelength of maximum reflection due to a physical,chemical or biological stimulus on each of the sensors, and/or maycooperate with external circuitry to provide a signal indicative of thestimulus for each of the sensors. Control block 49 may also control thelaser temperature by any of several known means and adjust the laserpower to provide a constant power output with respect to wavelengthusing an independent monitor detector 38 (FIG. 1).

[0040] In more detail, referring to FIG. 2A, diagnostic system 40includes a tunable VCSEL 30 which in this embodiment (FIG. 1) has a rearreflector stack of alternating quarter-wave layers of two differentmaterials 31, the Fabry-Perot cavity region that contains the activematerial 31 (here a solid optical cavity), and an upper reflector 32,made as movable, suspended mirror layers with different indices ofrefraction of transparent material on a cantilever as illustrated, or,alternately, as a reflective or partially reflective single layer, suchas aluminum. The relative position of the movable mirror structure withrespect to the rest of the structure is changeable by the application ofan electrostatic field or other control force, forming a variableoptical cavity 38 (here an air or vacuum optical cavity). The mirrorstructure could be made in a form of a diaphragm suspended by othermeans by selective etching and release techniques, the relative positionof which with respect to the rest of the structure is also changeable bythe application of an electrostatic force, magnetic force or otherforce. The result of this is that the effective optical distance betweenthe two reflectors making up the cavity 38 is adjustable. Since theresonant wavelength depends on this distance, the characteristicwavelength of the tunable VCSEL is continually tuned, for example, byvarying the applied voltage and thereby the electrostatic field betweenthe upper reflector and the remainder of the device.

[0041] It is desirable to provide energy within the tunable VCSEL 30 toachieve lasing. It should be noted that the energy could be provided byoptical pumping means or by electrical pumping means (p-n or p-i-njunction). Although both methods are suitable for a sensor system, theelectrically pumped embodiment is preferred from the point of view oflowest cost and greatest simplicity.

[0042] The operating wavelengths of the tunable VCSEL can be in thecommunication wavelength band (Chang-Hasnain [IEEE J. on Select. Topicsin Quantum Electronics, V 6, N 6, November 2000, p. 978] Vkhshoori[Electronics Letters, may 1999, V. 35, N.11 p. 900]) or around 960 nm(J. S. Harris, [Appl. Phys. Lett. 68 (7), February 1996 p. 891]) or inany other desired band in which VCSELs are produced. When the distancebetween the tunable VCSEL 30 and a Bragg grating or other type of sensor45 does not exceed about 1 km, many wavelength bands are usable. Whenthis distance exceeds about 1 km, the losses may become too high atwavelengths not in the communications bands and tunable VCSELs 30emitting within the communication wavelength bands may be more suitable.

[0043] A current control circuit within control block 49 (FIG. 2A)provides an electrical current to the tunable VCSEL 30, which controlsthe intensity of the output light. Adjusting the current through thediode (VCSEL active area 31) also causes slight changes in wavelength.However, this effect is not significant for this application. A pulsedcurrent can be used to cause pulsed light, which would be required forTime Division Multiplexing (TDM) (although it should be noted that TDMcould be realized by placing an electro-optical modulator anywherebetween tunable VCSEL 30 and sensor array 45). In addition, atemperature control circuit could be used in the illustrative embodimentto provide a current drive to a thermoelectric (TE) cooler to stabilizethe temperature of the tunable VCSEL 30 if needed. Other devices may beused to control the temperature if desired. A voltage control circuitcan be used to control the electrostatic force between the movablereflector 32 and the active layers of the tunable VCSEL 31 and, by suchmeans, can control the wavelength emitted by the tunable VCSEL in theillustrative embodiment. It should be noted that other controlmechanisms than electrostatic can be used to position the VCSEL tuningmirror, and the tuning signal may or many not be a voltage.

[0044] In all exemplary embodiments, the tunable VCSEL 30 can provide adivergent output light beam to either the end plane of fiber 41, placedin close vicinity to the tunable VCSEL and perpendicular to thedirection of emitted light propagation (butt-coupling method) or to afocusing lens, also represented by element 41, that provides focusedlight to optical fiber component isolator 42 or circulator 47. The lensmay instead be a lens system that provides this function. The lens alsocould be realized as a diffractive element 34 writtenphotolithographically on the surface of the VCSEL mirror, adjacent tothe fiber 41 or on the backside of the chip in the path of the lightbeam 36.

[0045] It should be noted that the optical circulator 47 may be replacedby an optical isolator 42 and a wavelength-independent two-way splitter,placed in line. This approach is less costly, although at least half ofthe optical power will be lost going each way. In addition, an opticalisolator could be placed between the tunable VCSEL 30 and the opticalcirculator 47 if very high suppression of back-reflected light isneeded. In this embodiment, the sensor can be a fiber Bragg grating, aplanar Bragg grating, a surface plasmon resonance sensor, a Fabry-Perotetalon sensor or a characteristic absorber/reflector sensor.

[0046] Additionally in the FIG. 2A embodiment, the light from thetunable VCSEL 30 propagates toward the sensor 45 that is composed of anarray of sensors disposed at intervals along the optical fiber 44. EachBragg grating sensor within sensor array 45 reflects a predeterminednarrow wavelength band of light and passes the remaining wavelengths ontoward the next sensor. The transmitted beam therefore contains a narrowabsorption band corresponding to the reflected band, but the remainderof the light is available to use with the other sensors. The sensors inarray 45, in the illustrative embodiment, can be placed in parallel orin series and multiplexed by Wavelength Division Multiplexing (WDM)and/or TDM. WDM is realized by sensors 45 having different centralreflection wavelengths, while TDM is realized by intentionallyintroduced time delays, by any technique known to those skilled in theart, between sensors that may or may not have the same centralreflection wavelengths. In this embodiment, the sensors can be fiberBragg gratings, planar Bragg gratings, surface plasmon resonancesensors, characteristic absorption/reflection sensors or Fabry-Perotetalon sensors.

[0047]FIG. 2B shows an example illustrative embodiment of a diagnosticsystem 40 provided in accordance with a second aspect of this invention.FIG. 2B is a schematic block diagram of a first state of anillustrative, exemplary sensor diagnostic system capable of determiningthe value of static and dynamic physical, chemical or biological stimuliin a transmission mode; necessarily employing a tunable VCSEL as anoptical excitation source and a photodiode or similar simple device as adetector. Preferred embodiment diagnostic system 40 includes aMEMS-tunable VCSEL 30, a fiber light coupling means 41, an opticalisolator 42, a wavelength reference 43, exterior fiber 44 and couplingmeans to the sensor or sensor array 45, exit fiber 46, a photodetector48, and a control block 49. In said illustrative example, a tunableVCSEL 30 is assembled with necessary means to fiber couple 41 theemitted light, isolate 42 the VCSEL from back-reflections, provide anaccurate wavelength reference 43 and couple the VCSEL assembly 40 to anexternal fiber 44 to convey the laser light to an optical sensor orsensor array 45 in a transmission mode. An exit optical fiber 46 isprovided to couple the optical output transmitted through the sensor orsensor array 45 to the photodetector 48, the electrical signal fromwhich is relayed to the control block circuitry 49 and externalelectronic circuitry as required. The control block may or may notcontrol the laser temperature via a thermoelectric element or othermeans and may or may not adjust the laser power output according to asignal from a monitor photodiode 38, as required.

[0048] In this embodiment, the sensor array 45 includes individual Bragggratings that each transmits light having different, non-overlapping,associated amplitude transmission minima at individual transmissionwavelengths, spectrally distinguishable one from the other. In theexemplary embodiment, the wavelength position of the amplitude minimumtransmitted by each of the Bragg gratings in the array 45 varies inresponse to a physical stimulus or perturbation imposed on thecorresponding sensor. The sensors in array 45, in the illustrativeembodiment, can be placed in parallel or in series and multiplexed byWavelength Division Multiplexing (WDM). TDM is not applicable in thisembodiment. Other aspects of this embodiment are the same as in thefirst embodiment, drawn in FIG. 2A. In this embodiment, the sensors canbe fiber Bragg gratings, planar Bragg gratings, surface plasmonresonance sensors, characteristic absorption/reflection sensors orFabry-Perot etalon sensors.

[0049] In preferred embodiments illustrated in FIGS. 2A and 2B, thefiber 44 and the sensor array 45 may be bonded to or embedded in astructure which is being monitored for a perturbation change, such asdynamic or static strain and/or temperature and/or pressure and/orelectrical current/or magnetic field. The structure may be made ofmetal, plastic, composite, or any other materials and the sensors may bedisposed on or within the structure.

[0050] Signal processing circuits (FIGS. 2A, 2B) analyze the electricalsignals and provide a plurality of output electrical perturbationsignals, indicative of the perturbation being measured by the sensorswithin the structure. It should be understood that a single line that istime multiplexed or that provides serial digital data for each sensormight also be used.

[0051] In the embodiments illustrated in FIGS. 2A and 2B, the wavelengthtuning control circuitry in control block 49 may include a functiongenerator in order to produce the control signal waveforms inillustrated in FIGS. 3A-3J. FIGS. 3A-3J are a series of graphs showingexemplary, illustrative time-varying tuning control signals, representedby V_(t), applied to a tunable VCSEL 30. Output wavelengths, λ, and theresulting optical power spectrum from the sensors 45 as a function ofboth time and wavelength in reflection and transmission modes are shownas well. The waveforms shown are a sawtooth waveform (3A-3D), sinusoidalwaveform (3E-3H) and triangular waveform (3I, 3J), but many others couldbe used. It is important to the sensor system operation that thewavelength versus time should be known accurately, and the lineartriangle wave (3I, 3J) would be superior from that point of view. Thetriangle waveform also allows reading all sensors 45 twice per cycle. Inthe exemplary embodiment, the control signal V_(t) relates directly tothe expansion or contraction of the cavity 38 in the VCSEL 30, therebycausing the wavelength λ of the output light to vary in proportion tothe applied control signal V_(t). Thus, the wavelength λ of the lightvaries linearly from λ₁ to λ₂, which range includes at least one peakreflection or transmission minimum wavelength λ_(b) from a sensor andoptionally at least one peak or minimum from a wavelength reference,which is shown as WR. For the sake of simplicity of the figures,reflection or transmission from just one sensor is shown. Also forsimplicity of illustration, the optical power graphs for the triangularwave are not shown, as they are similar to 3C and 3D with the sensorsignals occurring twice per cycle.

[0052] The triangle waveform (FIGS. 3I, 3J), although providing lineardependence of the wavelength vs. time, has the disadvantage of having adiscontinuity in the waveform that will by its nature induce higherfrequencies, or ringing, into the system. The ringing can be filteredout by various means known in the art, but a penalty is paid in time andefficiency. The sinusoidal control signal will provide frequencystability and power-conserving scanning with much faster scanning ratedue to the elimination of the stabilizing time required of a mechanicalstructure when a discontinuous forcing function is applied, such as thetriangle wave. With the sinusoidal waveform, the entire scan can occurin a few microseconds or shorter time. This is two orders of magnitudespeed advantage over conventional lasers, allowing better statisticalaveraging techniques to be used and allowing non-spectral shift sensors(e.g., bandgap semiconductor or other characteristicabsorber/reflectors) to be scanned fast enough to minimize optical noiseinterference. The optical power sensor signals for the sinusoidalwaveforms are illustrated schematically in FIGS. 3G and 3H, showing thenon-linear nature of the signals in time.

[0053] As a result of the scan through the wavelength range, the opticalsignal at the input to the optical detector 48 as well as the electricalsignal from the optical detector will appear as indicated inillustration FIG. 3C or 3G for the reflection mode and FIG. 3D or 3H forthe transmission mode. In particular, the electrical signal from theoptical detector will experience a sharp increase or decreaserespectively (as dictated by sensor design) in power centered at thecentral wavelength λ_(a) of each sensor. If TDM is also used, the outputis somewhat more complex, but know to those skilled in the art. For thesinusoidal drive, the results of a scan are given in FIGS. 3G, 3H.

[0054] In the illustrative embodiment, the control block 49, incooperation with additional signal processing circuitry, determines thestatic or dynamic value of the sensor stimulus by determining at whatwavelengths the maxima or minima in signal level occur and determiningthe amount of change from the wavelength maximal or minima of theunperturbed sensors. Calibration defines the relationship between achange in the stimulating parameter and a corresponding change inwavelength. The wavelength value is determined by monitoring thewavelength control signal and comparing it to the wavelength reference43 or mirror 32 position feedback 35 (capacitive), as required. Becausethis signal is directly related to the wavelength of the tunable VCSEL30, it provides a directly proportional value of the instantaneouswavelength. Many computation algorithms known to those skilled in theart can perform the determination of wavelength position of the minimaor maxima. For the illustrative purposes only, one of the possiblealgorithms is described below. The ability to calculate the position ofan extremum from relatively few data points enables enhanced accuracywith lower computational overhead.

[0055] During the each full tunable VCSEL wavelength scan N intensitymeasurement points are taken. Since photodetector 48 and the signalprocessor in the control block 49 can be made to operate by known art atGHz frequencies, the number N could be adequately large even if thetunable VCSEL could be operated at a maximum tuning speed of tens ofkHz. Let us assume that the tunable VCSEL operates at a frequency ofonly 1 kHz (each scan takes 1 millisecond), which is still two orders ofmagnitude faster than commonly available from non-MEMS tunable lasers.In this case, at least 100,000 intensity measurements could be takenduring each scan. Further assuming the maximum wavelength span of thetunable VCSEL does not exceed 50 nm, intensity measurements could bemade every 0.5 picometers. Since communications art has advanced intothe tens of GHz and slower scanning speeds can be tolerated in practicalsituations, greater wavelength resolution could be obtained. Further,since typical Bragg reflection peaks (or transmission minima) wavelengthwidths are on the order of hundreds of picometers, the embodimentsillustrated in FIGS. 2A and 2B are capable of extremely high wavelengthresolution. In the embodiment of Bragg gratings employing phase shifts,in which a much narrower peak or valley (20 picometers or narrower) isincorporated inside the primary valley or peak (respectively), thesensor data rate obtainable, as illustrated, will enable several datapoints to be taken within the phase shift band. This in turn will allowthe interpolation of the spectral data from the sensor array by amathematically smooth, continuous function of time, F(t). F(t) can thenbe transformed to a function of wavelength, F(X), according to the typeof VCSEL tuning drive used, or directly into a function of the parameterbeing measured. Many applicable mathematical techniques and theirelectronic implementations are known in the art. When used, wavelengthreferences can be analyzed in the same manner. Further simple algorithmsare used to compute wavelength changes for each sensor by comparison tothe previous F(λ) and the wavelength reference.

[0056] Instead of relying on the tuning control signal or feedback froma cantilever or diaphragm position monitoring means, such ascapacitance, to calibrate the VCSEL wavelength against time, anadditional unstrained or unperturbed reference means in the form of atleast one Bragg grating, Fabry-Perot etalon or absorption cell may beinserted into the optical path at 43. Said reference grating or cellmust cause at least one reflection peak or absorption valley within thetuning range and not interfering with any sensor wavelength band, andmay provide multiple extrema at λ_(ref 1), λ_(ref 2), λ_(ref 3), . . .λ_(ref n) that are always located at the same wavelength positions.Knowledge of the predetermined cycle rate, or waveform, of the voltageor other tuning signal, together with such reference wavelengths,provides the signal processing circuit with sufficient information tosynchronize the beginning of each new tuning cycle with the laserwavelength. The number of wavelength reference points is determined bythe accuracy and linearity of the laser tuning mechanism and therequired accuracy of the physical parameter measurement. The fewestreference points will provide the most economical system. In place of areference Bragg grating or gratings, a number of high finesseFabry-Perot cavity filters could be used. Another applicable method ofmaintaining wavelength accuracy would be to place an acetylene cell inthe optical path. Acetylene exhibits a number of very sharp absorptionpeaks in the communications wavelength bands that can be used tocalibrate the system on every cycle or every half cycle. Othertechniques may also be employed to maintain calibration accuracy toneeded levels by those skilled in the art.

[0057] Even though the embodiments have been most frequently describedas using Bragg gratings as the sensors that detect the environmentalstimulus, any reflective or transmittive device having a narrowreflection or transmission wavelength band, or transition slope (e.g.,bandgap semiconductors) or any other reflection or transmission spectralpeculiarity that shifts with applied perturbation may be used. Someexamples of such sensors include Fabry-Perot cavity pressure,temperature and/or displacement sensors, waveguide and surface plasmonresonance-based biological and/or chemical sensors, semiconductorbandgap strain, temperature or pressure sensors and fluorescent andvibronic materials. In the latter two types of materials, the absorptionbands are much narrower than that of semiconductors, and they do nothave to exhibit fluorescent light output in the spectral range of use.

[0058] An example of a surface plasmon resonance-based sensortransmission spectrum is given in FIG. 4. The valley in the curve,caused by a surface plasmon, is shifted in wavelength by, for example, abiomass specimen to be detected as it is adsorbed onto the sensorsurface. The absolute value of said shift provides very preciseinformation about the concentration of, for example, a reagent in thesolution. Additionally, the temporal behavior of said shift can provideinformation about the kinetics of associated chemical interactions. Thehigh tuning rate of the MEMStunable VCSEL enables faster and higherresolution kinetics studies possible, widening the range ofapplications. The employment of MEMS-tunable VCSEL 30 as a light sourcewill make the afore-described types of sensor systems considerably lessexpensive and more functional than those employing prior art lasersand/or optical spectrometers and will provide at least an order ofmagnitude increase in the system resolution through computational andstatistical means. Another feature of surface plasmon resonance sensorsis that the resonance transmission valley can be located at anypredetermined wavelength within the near infrared or infrared spectrum.Thus, tunable VCSELs 30 operated at 950-980 nm, for example, will beequally as useful as VCSELs operating in the communications bands in the1310 nm and 1550 nm ranges, with little change in cost. This furtheradvantage of tunable VCSEL sensor systems allows a wide variety ofsensor types and materials to be matched with a suitable, inexpensivetunable laser. The mathematical algorithm for extracting the position ofthe wavelength at the minimum of transmission will be similar to the onedescribed for the reflective or tranmissive Bragg grating sensor above.Wavelength multiplexing in this case will be limited by the greaterwidth (typically tens on nanometers) of SPR reflectivity minima and themaximum tuning wavelength span of the tunable VCSEL, possibly as low astwo sensors. Time division multiplexing is applicable in this case, aswell.

[0059] An illustrative example of the transmission spectrum of asemiconductor bandgap absorption-edge temperature sensor is shown inFIG. 5. The bandgap edge wavelength position is a function of thesemiconductor temperature according to well-known mathematical andphysical formulations. Such an absorption edge is blue-shifted as awhole when the temperature decreases and red-shifted when thetemperature increases, as illustrated in FIG. 5A. In this case, which isillustrative of its type, the scanned absorption edge will only yieldaccurate temperature or pressure data if the scanning process iscompleted in a much faster time than either the thermal response time ofthe semiconductor mass or the speed and/or the frequency ofabsorption-dependent noise (e.g., microbending noise) in the remainderof the fiber circuit. This is because the absorption of thesemiconductor cannot be distinguished from absorption noise unless theshape of the edge can be traced out very rapidly. For the greatestaccuracy, the spectral shape, undistorted by optical noise, of theabsorption curve is required, not the absolute value of the absorption.Thus an advantage of the exceptional tuning speed of the MEMs-tunedVCSEL 30 in this arrangement will make possible inexpensive fiber optictemperature sensors using chips of many semiconductor materials, with orwithout a mirrored surface, with good sensitivity and adequate accuracyfor applications such as microwave ovens. Further, such sensors can beselected to match the desired temperature range and VCSEL properties byusing alloy compositions available with continuously varying bandgaps,such as alloys and compounds of indium, aluminum, gallium and arsenic orsilicon and germanium.

[0060]FIGS. 5B and 5C are illustrative examples of improved means ofdetecting precisely the spectral position of an absorption band with avery wide maximum, such as the “long pass” band of a semiconductor. The“S”-shaped absorption curve can be converted to a peaked curve by takingthe first derivative, as shown in FIG. 5B. The computational algorithmfor this case is then similar to the cases of Bragg grating sensors andSPR-based sensors described heretofore. This is equally applicable to asingle pass of the light through the absorber in transmission, or adouble pass through the material if a mirror means is utilized on theoutput side of the sensor. A second means of accurately determining theamount of wavelength change from the unperturbed wavelength is providedby taking the second derivative of the transmission spectrum, as shownin FIG. 5C, which provides the opportunity to use a “zero cross-over”point to define the spectral shift. In the case of a pure semiconductor,the sensors will be self-calibrating since the wavelength dependence ofthe absorption edge is very well known. In the case of alloys,calibration may be performed. Many computation algorithms known to thoseskilled in the art can perform the determination of wavelength positionutilizing the peak of the first derivative or the zero crossing of thesecond derivative. The spectra can be interpolated, smoothed orsubjected to any other mathematical analysis known to those skilled inthe art.

[0061] Referring to Bragg grating sensors, the sensors 45 need not bewritten into the same type of fiber 44 as the fiber that feeds thesensors, e.g., the sensors can be spliced into the fiber 44 or they canbe separate planar chips, optically coupled to the fiber by meanscommonly known in the art.

[0062] Further, the embodiments have been described as employing anoptical fiber 44, but any other form of optical waveguide may be used ifdesired.

[0063] Also, it should be understood that the tuning control circuit 49and subsequent signal processing can be done with any degree ofcombination of software and hardware by many methods known in the art.

[0064] Although the invention has been described and illustrated withrespect to the exemplary embodiments thereof, it should be understood bythose skilled in the art that the foregoing and various other changes,omissions and additions may be made without departing from the scope ofthe invention.

We claim:
 1. An optical sensor diagnostic system, comprising: a tunableVCSEL incorporating an integrated MEMS wavelength tuner for providingwavelength-tunable light in response to a tuning control signal, saidtunable light being launched into an optical waveguide, said tunableVCSEL including a movable tuning mirror and a capacitive or opticaldetector for detecting the position of the movable tuning mirror andproviding feedback; at least one optical sensor, disposed in the path ofsaid tunable light, said at least one sensor providing a transmittedlight having at least one associated characteristic amplitude featureselected from the group consisting of a minimum, a maximum or a slopelocated at a particular wavelength within the transmitted wavelengthrange, said wavelength at each minimum, maximum or sloped transmissionamplitude being responsive to an environmental stimulus imposed uponsaid at least one sensor; said tunable VCSEL individually illuminatingsaid at least one sensor in a wavelength range spanning said wavelengthlocation of said associated characteristic transmission amplitudefeature; an optical isolator, disposed in the path of said tunable lightbetween said tunable VCSEL and said at least one sensor, for isolatingsaid tunable light source from light reflected from said at least onesensor; an optical detector, disposed in the path of said transmittedlight, for detecting said transmitted light from said at least onesensor and for providing an electrical detection signal indicative ofthe power of said transmitted light throughout a predeterminedwavelength range; a controller for providing a variable tuning controlsignal to said tunable VCSEL indicative of the desired wavelength ofsaid tunable light; at least one wavelength reference independent ofsaid tuning control signal and moveable mirror position detectordisposed in the path of the light; and a signal processor responsive tosaid electrical detection signal, for detecting a wavelength defined onthe characteristic transmission amplitude feature in order toquantitatively detect the effect on said at least one sensor due to saidenvironmental stimulus, changes in said wavelength at the characteristictransmission amplitude feature caused by changes in said environmentalstimulus, and for providing a signal indicative of said stimulus orchange therein.
 2. The optical sensor diagnostic system of claim 1wherein said at least one sensor comprises plural wavelength divisionmultiplexed optical sensors.
 3. The optical sensor diagnostic system ofclaim 1 wherein said at least one sensor comprises plural time divisionmultiplexed optical sensors.
 4. The optical sensor diagnostic system ofclaim 1 wherein said at least one sensor comprises an environmentreference or compensation sensor.
 5. The optical sensor diagnosticsystem of claim 1 wherein said detection and signal processor comprisesa tracker, responsive to said electrical detection signal, for adjustinga voltage or other control signal to allow said tunable light to trackstatic and dynamic values of said characteristic transmission amplitudefeature for said at least one sensor, thereby providing utilization ofthe control signal as the output characteristic of the physicalstimulus, making unnecessary scanning of the complete wavelength rangeand greatly increasing the speed of data acquisition.
 6. The opticalsensor diagnostic system of claim 1 wherein said controller comprises amodulator for modulating said voltage control signal at a predeterminedmodulation frequency.
 7. The optical sensor diagnostic system of claim 1wherein said signal processor comprises a demodulator operating at saidmodulation frequency, for demodulating said electrical detection signaland for providing a demodulated signal indicative thereof.
 8. Theoptical sensor diagnostic system of claim 1 wherein said signalprocessor incorporates a computational element for increasing theaccuracy and precision of determining the wavelength position of saidcharacteristic transmission amplitude feature and changes therein foreach of said sensor.
 9. The optical sensor diagnostic system of claim 1wherein: said at least one sensor comprises plural sensors; saidcontroller comprises a scanner that scans said control signal for thepurpose of causing said tunable VCSEL to scan its wavelengths acrosssaid characteristic transmission amplitude features of said pluralsensors; and said signal processor determines, in response to saidvoltage or other control signal, the wavelength of said tunable lightfrom the magnitude of said voltage or other control signal and/or mirrorposition feedback signal and for determining which of said pluralsensors is being illuminated, thereby determining the value of theenvironmental stimulus at the position of said illuminated sensor. 10.The optical sensor diagnostic system of claim 1 wherein: said at leastone sensor comprises plural sensors; said controller comprises a scannerthat scans said control signal so as to cause said tunable VCSEL to scanacross the characteristic transmission amplitude features of said pluralsensors and for providing a synchronization signal indicative of whensaid voltage control signal begins and ends said scanning; and saidsignal processor determines, in response to said synchronization signal,which of said plural sensors is being illuminated, thereby determiningchanges in said wavelength at said characteristic transmission amplitudefeature.
 11. The optical sensor diagnostic system of claim 1 whereinsaid at least one sensor comprises at least one fiber or planar Bragggrating.
 12. The optical sensor diagnostic system of claim 1 whereinsaid at least one sensor includes at least one Bragg grating thatincorporates phase shift in its structure, said phase shift producing asharper maximum within said transmitted wavelength band minimum.
 13. Theoptical sensor diagnostic system of claim 1 wherein said at least onesensor comprises at least one Fabry-Perot etalon.
 14. The optical sensordiagnostic system of claim 1 wherein said at least one sensor comprisesat least one Surface Plasmon Resonance structure.
 15. The optical sensordiagnostic system of claim 1 wherein said at least one sensor comprisesat least one thin film or bulk material characteristic absorbermaterial.
 16. The system of claim 15 wherein characteristic absorbermaterial comprises one or more vibronic, excitonic or fluorescentmaterials.
 17. The optical sensor diagnostic system of claim 1 whereinsaid environmental stimulus comprises any combination of mechanicalstress, temperature, pressure, electrical current, electrical field,magnetic field or chemical or biological material on said sensor. 18.The optical sensor diagnostic system of claim 1 wherein at least onewavelength reference, not affected by any environmental stimulus,comprising at least one of the group of a Bragg grating, a phase shiftBragg grating, a Fabry-Perot etalon or a gas-containing chamber, isdisposed in the optical path.
 19. The optical sensor diagnostic systemof claim 1 wherein the wavelength reference comprises at least onegas-containing chamber containing acetylene gas.
 20. An optical sensordiagnostic system, comprising: a VCSEL incorporating integrated MEMSwavelength tuner for providing wavelength-tunable light in response to atuning control signal, said tunable light being launched into an opticalwaveguide, wherein is provided at least one optical sensor, disposed inthe path of said tunable light, each providing a reflected light havingat least one associated characteristic amplitude feature from but, notlimited to, the group, a minimum, a maximum or a slope located at aparticular wavelength within the reflected wavelength range, saidwavelength at each minimum, maximum or sloped reflection amplitude beingresponsive to an environmental stimulus imposed upon a correspondingsensor; said tunable VCSEL for individually illuminating each of saidsensor in a wavelength range spanning said wavelength location of saidassociated characteristic reflection amplitude feature; opticaldetector, disposed in the path of said reflected light, for detectingsaid reflected light from each of said sensor and for providing anelectrical detection signal indicative of the power of said reflectedlight throughout the appropriate wavelength range; optical circulator,disposed in the path of said tunable light between said tunable VCSELand said sensor, for isolating said tunable light source from lightreflected from said sensor and directing the light to said detector;voltage or other controller for providing a variable tuning controlsignal to said tunable VCSEL indicative of the desired wavelength ofsaid tunable light; capacitive or optical detector that detects theposition of the movable tuning mirror and providing feedback; and atleast one wavelength reference independent of said tuning control signaland moveable mirror position detector disposed in the path of the light,and signal processor responsive to said electrical detection signal, fordetecting a wavelength defined on the characteristic reflectionamplitude feature in order to quantitatively detect the effect on saidsensor due to said environmental stimulus, changes in said wavelength atthe characteristic reflection amplitude feature caused by changes insaid environmental stimulus, and for providing a signal indicative ofsaid stimulus or change therein for each of said sensor.
 21. The opticalsensor diagnostic system of claim 20 wherein said optical sensors arewavelength division multiplexed.
 22. The optical sensor diagnosticsystem of claim 20 wherein said optical sensors are time divisionmultiplexed.
 23. The optical sensor diagnostic system of claim 20wherein at least one of said sensors serves as an environment referenceor compensation sensor.
 24. The optical sensor diagnostic system ofclaim 20 wherein said detection and signal processor comprises tracker,responsive to said electrical detection signal, for adjusting saidvoltage or other control signal to allow said tunable light to trackstatic and dynamic values of said characteristic reflection amplitudefeature for each of said sensor, thereby providing utilization of thecontrol signal as the output characteristic of the physical stimulus,making unnecessary scanning of the complete wavelength range and greatlyincreasing the speed of data acquisition.
 25. The optical sensordiagnostic system of claim 20 wherein said voltage controller comprisesa modulator for modulating said voltage control signal at apredetermined modulation frequency.
 26. The optical sensor diagnosticsystem of claim 20 wherein said signal processor comprises a demodulatoroperating at said modulation frequency, for demodulating said electricaldetection signal and for providing a demodulated signal indicativethereof.
 27. The optical sensor diagnostic system of claim 20 whereinsaid signal processor performs computations that increase the accuracyand precision of determining the wavelength position of saidcharacteristic reflection amplitude feature and changes therein for eachof said sensor.
 28. The optical sensor diagnostic system of claim 20wherein: said voltage or other controller comprises a scanner that scanssaid voltage control signal so as to cause said tunable VCSEL to scanits wavelength across said characteristic reflection amplitude featureof any or all of said sensor; and said signal processor determines, inresponse to said voltage or other control signal, the wavelength of saidtunable light from the magnitude of said voltage or other control signaland/or mirror position feedback signal and for determining which of saidsensor is being illuminated, thereby determining the value of theenvironmental stimulus at the position of said individual sensor. 29.The optical sensor diagnostic system of claim 20 wherein: said voltageor other controller comprises a scanner that scans said voltage controlsignal so as to cause said tunable VCSEL to scan across the wavelengthsof the characteristic reflection features of all of said sensor; and forproviding a synchronization signal indicative of when said voltagecontrol signal begins said scanning; and said signal processordetermines, in response to said synchronization signal, which of saidsensor is being illuminated, thereby determining changes in saidwavelength at said characteristic reflection amplitude feature.
 30. Theoptical sensor diagnostic system of claim 20 wherein said at least onesensor comprises at least one fiber or planar Bragg grating.
 31. Theoptical sensor diagnostic system of claim 30 wherein at least one Bragggrating of at least one sensor comprises at least one incorporated phaseshift in its structure, said phase shift producing a sharper minimumwithin said reflected wavelength band maximum.
 32. The optical sensordiagnostic system of claim 20 wherein said at least one sensor comprisesat least one Fabry-Perot etalon.
 33. The optical sensor diagnosticsystem of claim 20 wherein said at least one sensor comprises at leastone Surface Plasmon Resonance structure.
 34. The optical sensordiagnostic system of claim 20 wherein at least one sensor is disposed ina branch waveguide or optical fiber coupled to the main trunk waveguideby a coupler.
 35. The optical sensor diagnostic system of claim 34wherein said at least one sensor comprises at least one thin film orbulk material characteristic absorber material.
 36. The characteristicabsorber material of claim 35 comprising at least one semiconductor. 37.The characteristic absorber material of claim 36 chosen from the fullpossible range of alloys and compounds of zinc, cadmium, mercury,silicon, germanium, tin, lead, aluminum, gallium, indium, bismuth,nitrogen, oxygen, phosphorus, arsenic, antimony, sulfur, selenium andtellurium.
 38. The characteristic absorber material of claim 35comprises one or more vibronic, excitonic or fluorescent materials. 39.The characteristic absorber material of claim 35 wherein said sensorcomprised of said characteristic absorber material incorporates a mirrorat the distal end, providing signal reflection by double-passtransmission.
 40. The optical sensor diagnostic system of claim 20wherein at least one sensor produces a characteristic absorption featurein the form of a slope, wherein: the said wavelength indicative of thecharacteristic absorption slope is determined by taking the firstderivative of the light amplitude with respect to the wavelength, and byanalytically extracting the wavelength position of resulting said firstwavelength derivative extremum, or, alternatively, the said wavelengthindicative of the characteristic absorption slope is determined bytaking the second derivative of the light amplitude with respect to thewavelength and by analytically extracting the wavelength positions ofsaid second wavelength derivative zeros.
 41. The optical sensordiagnostic system of claim 20 wherein said environmental stimulus is anycombination of mechanical stress, temperature, pressure, electricalcurrent, electrical field, magnetic field or chemical or biologicalmaterial on said sensor
 42. The optical sensor diagnostic system ofclaim 20 wherein at least one wavelength reference, not affected by anyenvironmental stimulus, comprising at least one of the group of a Bragggrating, a phase shift Bragg grating, a Fabry-Perot etalon or agas-containing chamber, is disposed in the optical path.
 43. The opticalsensor diagnostic system of claim 42 wherein the gas-containing chambercontains acetylene gas.
 44. The optical sensor diagnostic system ofclaim 20 wherein a said sensor comprising an optical fiber having a corewaveguide and a cladding or cladding/buffer layer surrounding the corewaveguide, in addition incorporating an input/output end and a terminalreflection end, wherein the terminal reflection end is defined by an endface of the core waveguide in contact with a mirrored layer such thatthe light is caused to reverse its direction of propagation and exitsthe input/output end. In this embodiment, the majority of the opticalfiber length does not support surface plasmon resonance, but instead theoptical fiber incorporates a sensing area located between theinput/output end and terminal reflection end or at the terminalreflection end. Said sensing area is defined by a surface plasmonresonance-supporting metal in contact with at least a portion of thesurface of the optical fiber core waveguide free from the surroundingcladding or cladding/buffer layer.
 45. The optical sensor diagnosticsystem of claim 44 wherein the said sensing area further contains atleast one additional functional layer adhered to the surface plasmonresonance-supporting metal layer.
 46. The optical sensor diagnosticsystem of claim 45 wherein the at least one additional layer comprises achemically reactive layer.
 47. The optical sensor diagnostic system ofclaim 45 wherein the at least one additional layer comprises abiologically reactive layer.
 48. The optical sensor diagnostic system ofclaim 44 wherein the said plasmon resonance supporting metal is one ormore layers of elements or alloys chosen from the group silver, gold,copper or aluminum.
 49. The optical fiber sensor according to claim 44further incorporating a polarizer positioned anywhere between saidtunable VCSEL and said sensor, said polarizer selecting light withpolarization P.
 50. The optical fiber sensor according to claim 45further comprising a first polarizer positioned between the said tunableVCSEL and said circulator, said polarizer selecting light withpolarization state between S and P polarizations.
 51. The optical fibersensor according to claim 50 further comprising a second polarizerpositioned between the said sensor and said detector, said secondpolarizer being oriented with respect to the said first polarizer suchthat a phasepolarization enhancement is obtained of the ratio of thepower amplitudes at wavelengths outside the said minimum of reflectionfeature to the power amplitude at the exact minimum of reflection. 52.In a sensing system of the type including an optical fiber sensor oroptical waveguide having a radiation reflectance or transmissivitycharacteristic that varies in response to a stimulus, an optical pathbeing defined between a coupler and said optical fiber sensor or opticalwaveguide, a sensing method comprising: (1) operating a Vertical CavitySurface Emitting Laser to generate radiation; (2) tuning the VerticalCavity Surface Emitting Laser to vary the wavelength of said VerticalCavity Surface Emitting Laser-generated radiation; (3) coupling at leastsome of the radiation emitted by said Vertical Cavity Surface EmittingLaser to said coupler; and (4) analyzing radiation transmitted orreflected by said sensor or waveguide for variations of saidcharacteristic caused by said stimulus.
 53. A sensing system includingan optical fiber sensor or optical waveguide, said system comprising: acoupler being coupled to the optical fiber sensor or optical waveguidehaving a wavelength-selective radiation transrnissivity characteristicthat varies in response to a stimulus; a Vertical Cavity SurfaceEmitting Laser coupled to said coupler, said Laser operated to generateradiation and supply said radiation to said coupler; a tuning devicethat tunes the Vertical Cavity Surface Emitting Laser to vary thewavelength of said Vertical Cavity Surface Emitting Laser-generatedradiation; a detector that detects radiation transmitted or reflected bythe optical fiber sensor or waveguide; and an analyzer that analyzessaid detected radiation for at least one variation caused by saidstimulus.